Eye Therapy System

ABSTRACT

Embodiments apply a cross-linking agent to a region of corneal tissue. The cross-linking agent improves the ability of the corneal tissue to resist undesired structural changes. For example, the cross-linking agent may be Riboflavin or Rose Bengal, and the initiating element may be photoactivating light, such as ultraviolet (UV) light. In these embodiments, the photoactivating light initiates cross-linking activity by irradiating the applied cross-linking agent to release reactive oxygen radicals in the corneal tissue. The cross-linking agent acts as a sensitizer to convert O 2  into singlet oxygen which causes cross-linking within the corneal tissue. The rate of cross-linking in the cornea is related to the concentration of O 2  present when the cross-linking agent is irradiated with photoactivating light. Accordingly, the embodiments control the concentration of O 2  during irradiation to increase or decrease the rate of cross-linking and achieve a desired amount of cross-linking.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 61/253,736, filed Oct. 21, 2009, the contents of which are incorporated entirely herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to the field of conducting eye therapy, and more particularly, to systems and methods for stabilizing changes to corneal tissue as a part of eye therapy.

2. Description of Related Art

A variety of eye disorders, such as myopia, keratoconus, and hyperopia, involve abnormal shaping of the cornea. Laser-assisted in-situ keratomileusis (LASIK) is one of a number of corrective procedures that reshape the cornea so that light traveling through the cornea is properly focused onto the retina located in the back of the eye. During LASIK eye surgery, an instrument called a microkeratome is used to cut a thin flap in the cornea. The corneal flap is then peeled back and the underlying corneal tissue is ablated to the desired shape with an excimer laser. After the desired reshaping of the cornea is achieved, the corneal flap is put back in place and the surgery is complete.

In another corrective procedure that reshapes the cornea, thermokeratoplasty provides a noninvasive procedure that applies electrical energy in the microwave or radio frequency (RF) band to the cornea. In particular, the electrical energy raises the corneal temperature until the collagen fibers in the cornea shrink at about 60° C. The onset of shrinkage is rapid, and stresses resulting from this shrinkage reshape the corneal surface. Thus, application of energy according to particular patterns, including, but not limited to, circular or annular patterns, causes aspects of the cornea to flatten and improves vision in the eye.

The success of procedures, such as LASIK or thermokeratoplasty, in addressing eye disorders, such as myopia, keratoconus, and hyperopia, depends on whether the desired reshaping of the cornea has been sufficiently stabilized.

SUMMARY OF THE INVENTION

Embodiments according to aspects of the present invention provide systems and methods for stabilizing corneal tissue and improving its biomechanical strength. For example, the embodiments may be employed to preserve the desired reshaping of corneal tissue produced by eye therapies, such as thermokeratoplasty or LASIK surgery.

In particular, the embodiments apply a cross-linking agent to a region of corneal tissue. The cross-linking agent improves the ability of the corneal tissue to resist undesired structural changes. For example, the cross-linking agent may be Riboflavin or Rose Bengal, and the initiating element may be photoactivating light, such as ultraviolet (UV) light. In these embodiments, the photoactivating light initiates cross-linking activity by irradiating the applied cross-linking agent to release reactive oxygen radicals in the corneal tissue. The cross-linking agent, e.g. Riboflavin or Rose Bengal, acts as a sensitizer to convert O₂ into singlet oxygen which causes cross-linking within the corneal tissue.

The rate of cross-linking in the cornea is related to the concentration of O₂ present when the cross-linking agent is irradiated with photoactivating light. Accordingly, aspects of the present invention control the concentration of O₂ during irradiation to increase or decrease the rate of cross-linking and achieve a desired amount of cross-linking.

To increase the presence of O₂ during irradiation, the cross-linking agent in some embodiments may be saturated or supersaturated with O₂ before application to the cornea.

In other embodiments, a steady state of O₂ may be maintained above the eye to expose the cornea to higher concentrations of O₂ during irradiation.

In further embodiments, a gel, such as a methylcellulose gel, may be saturated or supersaturated with O₂. The gel acts as a carrier for O₂. The gel may then be applied to the cornea after the cross-linking agent has been applied to the cornea. Alternatively, the gel may be mixed with the cross-linking agent before the cross-linking agent is applied to the cornea.

In some embodiments, the rate of cross-linking may be monitored in real time and the concentration of O₂ may be dynamically increased or decreased to achieve a desired amount of cross-linking. Thus, embodiments include a system that provides a first amount of O₂ above the eye to introduce O₂ to the corneal tissue and expose the cornea to a first concentration of O₂ during irradiation. Based on the feedback from the real time monitoring, the system can then provide a second amount of O₂ above the eye to introduce another amount of O₂ to the corneal tissue and expose the cornea to a second concentration of O₂ during irradiation. The first amount of O₂ may be greater than the second amount of O₂, or vice versa. Changing the cornea's exposure from the first concentration to the second concentration changes the rate of cross-linking in the corneal tissue. Further changes to the concentration of O₂ during irradiation may be effected to control the rate of cross-linking. When necessary, the amount of O₂ above the eye may be substantially reduced to zero to prevent further introduction of O₂ to the corneal tissue during irradiation.

Other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, by illustrating a number of exemplary embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. The invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a high resolution image of a cornea after energy has been applied.

FIG. 1B illustrates another high resolution images of the cornea of FIG. 1A.

FIG. 1C illustrates a histology image of the cornea of FIG. 1A.

FIG. 1D illustrates another histology image of the cornea of FIG. 1A.

FIG. 2A illustrates an example approach for stabilizing or strengthening corneal tissue by applying a cross-linking agent according to aspects of the present invention.

FIG. 2B illustrates an example approach for stabilizing or strengthening corneal tissue by applying Riboflavin as a cross-linking agent according to aspects of the present invention.

FIG. 3A illustrates an example device that may be employed to supersaturate a cross-linking agent with O₂ according to aspects of the present invention.

FIG. 3B illustrates an example approach for stabilizing or strengthening corneal tissue by applying supersaturated Riboflavin as a cross-linking agent according to aspects of the present invention.

FIG. 4A illustrates an example device that may be employed to supersaturate a carrier gel with O₂ according to aspects of the present invention.

FIG. 4B illustrates an example approach for stabilizing or strengthening corneal tissue by mixing Riboflavin with a gel supersaturated with O₂ according to aspects of the present invention.

FIG. 4C illustrates an example approach for stabilizing or strengthening corneal tissue by applying a gel supersaturated with O₂ according to aspects of the present invention.

FIG. 5A illustrates an example device that may be employed maintain a steady state of O₂ above the eye to expose the cornea to higher concentrations of O₂ according to aspects of the present invention.

FIG. 5B illustrates another example device that may be employed to maintain a steady state of O₂ above the eye to expose the cornea to higher concentrations of O₂ according to aspects of the present invention.

FIG. 5C illustrates an example approach for stabilizing or strengthening corneal tissue by applying a state of O₂ above the eye to expose the cornea to higher concentrations of O₂.

FIG. 6 illustrates an example approach for stabilizing or strengthening corneal tissue by monitoring cross-linking activity in real time and controlling the amount of O₂ exposure to achieve desired rates of cross-linking according to aspects of the present invention.

DETAILED DESCRIPTION

Embodiments according to aspects of the present invention provide systems and methods for stabilizing corneal tissue and improving its biomechanical strength. For example, the embodiments may be employed to preserve the desired reshaping of corneal tissue produced by eye therapies, such as thermokeratoplasty or LASIK surgery.

FIGS. 1A-D illustrate an example of the effect of applying heat to corneal tissue with thermokeratoplasty. In particular, FIGS. 1A and 1B illustrate high resolution images of cornea 2 after heat has been applied. As FIGS. 1A and 1B show, a lesion 4 extends from the corneal surface 2A to a mid-depth region 2B in the corneal stroma 2C. The lesion 4 is the result of changes in corneal structure induced by the application of heat as described above. These changes in structure result in an overall reshaping of the cornea 2. It is noted that the application of heat, however, has not resulted in any heat-related damage to the corneal tissue.

As further illustrated in FIGS. 1A and 1B, the changes in corneal structure are localized and limited to an area and a depth specifically determined by the controlled application of heat. FIGS. 1C and 1D illustrate histology images in which the tissue shown in FIGS. 1A and 1B has been stained to highlight the structural changes induced by the heat. In particular, the difference between the structure of collagen fibrils in the mid-depth region 2B where heat has penetrated and the structure of collagen fibrils outside the region 2B is clearly visible. Thus, the collagen fibrils outside the region 2B remain generally unaffected by the application of heat, while the collagen fibrils inside the region 2B have been rearranged and formed new bonds to create completely different structures. In other words, unlike processes, such as orthokeratology, which compress areas of the cornea to reshape the cornea via mechanical deformation, the collagen fibrils in the region 2B are in an entirely new state. Treatment of the cornea produces structural changes to the stroma 2C, and the optomechanical properties of the cornea change under stress. Such changes include: straightening out the waviness of the collagen fibrils; slippage and rotation of individual lamellae; and breakdown of aggregated molecular superstructures into smaller units.

Although treatments, such thermokeratoplasty, may initially achieve desired reshaping of the cornea, the desired effects of reshaping the cornea may be mitigated or reversed at least partially if the collagen fibrils continue to change after the desired reshaping has been achieved. Therefore, aspects of the present invention provide approaches for preserving the desired corneal structure and shape that result from an eye therapy, such as thermokeratoplasty. In general, embodiments provide approaches for initiating molecular cross-linking of the corneal collagen to stabilize the corneal tissue and improve its biomechanical strength.

Referring to FIG. 2A, a treatment, such as thermokeratoplasty or LASIK surgery, is applied in step 210 to generate structural changes in the cornea and produce a desired shape change. In step 220, the corneal tissue is treated with a cross-linking agent 222. The cross-linking agent may be applied directly on the treated tissue and/or in areas around the treated tissue. In some embodiments, the cross-linking agent may be an ophthalmic solution that is broadly delivered by a dropper, syringe, or the like. Alternatively, the cross-linking agent may be selectively applied as an ophthalmic ointment with an appropriate ointment applicator. The cross-linking agent 222 is then activated in step 230 with an initiating element 232. Activation of the cross-linking agent 222, for example, may be triggered thermally by the application of microwaves or light from a corresponding energy or light source. The resulting cross-linking between collagen fibrils provides resistance to changes in corneal structure.

As described previously with reference to FIGS. 1A-D, for example, the lesion 4 extends from the corneal surface 2A to a mid-depth region 2B in the corneal stroma 2C. In such cases, the application of the cross-linking agent in step 220 introduces sufficient amounts of cross-linking agent to mid-depth regions of the corneal tissue where stronger and more stable structure is desired.

As FIG. 2B shows further, Ribloflavin is applied as a cross-linking agent 222′ to the corneal tissue in step 220. In addition, light from a ultraviolet (UV) light source may be applied as an initiating element 232′ in step 230 to initiate cross-linking in the corneal areas treated with Ribloflavin. Specifically, the UV light initiates cross-linking activity by causing the applied Riboflavin to release reactive oxygen radicals in the corneal tissue. The Riboflavin acts as a sensitizer to convert O₂ into singlet oxygen which causes cross-linking within the corneal tissue.

In human tissue, O₂ content is very low compared to the atmosphere. The rate of cross-linking in the cornea, however, is related to the concentration of O₂ when it is irradiated with photoactivating light. Therefore, it may be advantageous to increase or decrease the concentration of O₂ actively during irradiation to control the rate of cross-linking until a desired amount of cross-linking is achieved.

An approach according to aspects of the present invention involves supersaturating the Riboflavin with O₂. Thus, when the Riboflavin is applied to the eye, a higher concentration of O₂ is delivered directly into the cornea with the Riboflavin and affects the conversion of O₂ into singlet oxygen when the Riboflavin is exposed to the photoactivating light. As illustrated in FIG. 3A, the Riboflavin 222′ may be stored in a closed vessel, e.g., a vial, 300 under increased O₂ pressure 305. The increased O₂ pressure 305 results in a higher equilibrium concentration of O₂ in the Riboflavin 222′. The walls 310 of the vessel 300 are preferably opaque or otherwise prevent visible, UV, or other light from entering the vessel interior 301 to minimize the degradation of the Riboflavin 222′. Accordingly, referring to FIG. 3B, the step 215 supersaturates the Riboflavin 222′ with O₂ so that a supersaturated Riboflavin 222′ is applied in step 220.

According to other aspects of the present invention, rather than supersaturating the Riboflavin 222′ with O₂, another substance, such as a gel (e.g., a methylcellulose gel), may be saturated or supersaturated with O₂. As illustrated in FIG. 4A, a gel 421 may be stored in an interior 401 of a closed vessel, e.g., a vial, 400 under increased O₂ pressure 405. The increased O₂ pressure 405 results in a higher equilibrium concentration of O₂ in the gel 421. The gel can then act as a carrier for O₂.

Referring to FIG. 4B, step 216 saturates a gel 421 with O₂, and step 217 mixes the supersaturated gel 421 with the Riboflavin 222′, so that a mixture 422 containing the Riboflavin 222′ and the supersaturated gel 421 is applied in step 220. Alternatively, referring to FIG. 4C, step 216 saturates a gel 421 with O₂, and step 225 applies the gel 421 to the cornea after the Riboflavin 222′ has been applied to the cornea. In both FIGS. 4A and 4B, the gel 421 increases the presence of O₂ when the Riboflavin 222′ is activated with the UV light.

According to additional aspects of the present invention, a steady state of O₂ may be maintained at the surface of the cornea to expose the cornea to a selected amount of O₂ and cause O₂ to enter the cornea. The photoactivating light can then be applied to a cornea with the desired O₂ content.

As shown in FIG. 5A, a ring 500 is placed on the eye 1 to supply O₂ to the cornea 2 during irradiation. The ring 500 includes one or more ports 502 that direct a steady flow of O₂ to the cornea 2, which has been treated by Riboflavin. The flow applies O₂ at high pressure against the cornea 2, so that more O₂ is available during the irradiation of the Riboflavin in the corneal tissue. The ring 500 may optionally be held in place by suction.

As FIG. 5B illustrates, in another embodiment, an enclosure 510 receiving a supply of O₂ through a port 512 is placed on the eye to establish a steady state of O₂. The enclosure 510 may be held in place by a suction ring 512. As shown in FIG. 5B, the enclosure 510 may be a cup-like structure. The enclosure 510 maintains the O₂ at a higher pressure, e.g., higher than ambient, against the surface of the cornea 2. The concentration of O₂ within the enclosure 510 and above the surface of the cornea 2 can approach 100%. The O₂ within the enclosure 510 makes more O₂ to be available for the irradiation of the Riboflavin in the corneal tissue. At least a portion of the walls 514 of the enclosure 510 may be translucent to allow photoactivating light to pass through the enclosure 510 to irradiate the cornea 2 and activate the Riboflavin applied to the cornea 2. Alternatively, the light source may be disposed within the enclosure. The enclosure 510 may also include a valve that allows the gas to be released.

Accordingly, referring to FIG. 5C, step 227 establishes a steady state of O₂ above the corneal surface before the photoactivating light 232′ is applied in step 230 to initiate cross-linking with the Riboflavin 222′.

Referring to FIG. 6, the rate of cross-linking may be monitored in real time and the concentration of O₂ may be dynamically increased or decreased to achieve a desired amount of cross-linking. As FIG. 6 illustrates, corneal tissue is treated with Riboflavin 222′ in step 220. In step 228, a first amount of O₂ is provided above the corneal surface to introduce O₂ to the corneal tissue and establish a first concentration of O₂ in the cornea during irradiation. The devices described with reference to FIGS. 5A and 5B may be employed to change the amount of O₂ is provided above the corneal surface. The Riboflavin 222′ is then activated in step 230 with UV light 232′.

In step 240, the amount of cross-linking resulting from the activation of the Riboflavin 222′ is monitored. One technique for monitoring the cross-linking employs polarimetry to measure corneal birefringence and to determine the structure of the corneal tissue. In particular, the technique measures the effects of cross-linking on corneal structure by applying polarized light to the corneal tissue. The corneal stroma is anisotropic and its index of refractions depends on direction. The cornea behaves like a curved biaxial crystal with the fast axis orthogonal to the corneal surface and the slow axis (or corneal polarization axis) tangential to the corneal surface. Accordingly, a light beam emerging from the living eye after a double pass through the ocular optics contains information on the polarization properties of the ocular structures (except optically inactive humours). The technique of using birefringence to monitor the structural changes resulting from cross-linking is described further in U.S. Provisional Patent Application No. 61/388,963, filed Oct. 1, 2010, the contents of which are entirely incorporated herein by reference. A controller, employing conventional computer hardware or similar processing hardware, can be used to monitor the amount of cross-linking. Such hardware may operate by reading and executing programmed instructions that are stored or fixed on computer-readable media, such as conventional computer disk. In addition to being coupled to monitoring hardware, the controller may be coupled to, and automatically control, the device(s) that provide the O₂ above the corneal surface.

Based on the information from the real time monitoring in step 240, step 250 provides a second amount of O₂ above the eye to introduce another amount of O₂ to the corneal tissue and expose the cornea to a second concentration of O₂ during irradiation with UV light 232′ in step 260. Steps 240, 250, and 260 may be repeated any number of times to change the concentration of O₂ during irradiation to control the rate of cross-linking dynamically.

The first amount of O₂ in step 228 may be greater than the second amount of O₂ in step 250, or vice versa. Changing the cornea's exposure from the first concentration to the second concentration changes the rate of cross-linking in the corneal tissue as desired. If the information from step 240 indicates that the first amount of O₂ is too low, step 250 provides a second amount of O₂ that is greater than the first amount of O₂. On the other hand, if the information from step 240 indicates that the first amount of O₂ is too high, step 250 provides a second amount of O₂ that is greater than the first amount of O₂. It may be necessary to remove the first amount of O₂, e.g., from the enclosure 510, before providing the second amount of O₂ in step 250.

In some cases, it may be desired to provide substantially zero O₂ in step 250 to minimize or reduce the amount of O₂ in the corneal tissue during irradiation in step 260. Accordingly, step 250 may introduce a non-O₂ element or substance above the corneal surface. For example, nitrogen gas (N₂) may replace the O₂ supplied by the devices 500 and 510 shown in FIGS. 5A and 5B.

Although the embodiments described above may employ Riboflavin as a cross-linking agent, it is understood that other substances may be employed as a cross-linking agent. Thus, an embodiment may employ Rose Bengal (4,5,6,7-tetrachloro-2′,4′,5′,7′-tetraiodofluorescein) as a cross-linking agent (similar to the embodiment of FIG. 3B). Rose Bengal has been approved for application to the eye as a stain to identify damage to conjunctival and corneal cells. However, Rose Bengal can also initiate cross-linking activity within corneal collagen to stabilize the corneal tissue and improve its biomechanical strength. Like Riboflavin, photoactivating light 332′ may be applied to initiate cross-linking activity by causing the Rose Bengal to convert O₂ in the corneal tissue into singlet oxygen. The photoactivating light 332′ may include, for example, UV light or green light.

Thus, with Rose Bengal, the rate of cross-linking in the cornea is related to the concentration of O₂ when it is irradiated with photoactivating light. Therefore, it may be advantageous to increase or decrease the concentration of O₂ during irradiation to control the rate of cross-linking and achieve the desired cross-linking. The concentration of O₂ may be increased or decreased according to the techniques described previously. For example, the Rose Bengal may be saturated or supersaturated with O₂ before application to the cornea. Additionally or alternatively, a steady state of O₂ may be maintained above the eye to expose the cornea to higher concentrations of O₂ and cause O₂ to enter the cornea. In general, the O₂ content in the cornea may be controlled for more effective cross-linking for any agent that operates to produce a reactive oxygen species for cross-linking.

Although aspects of the present invention have been described in connection with thermokeratoplasty or LASIK surgery, it is understood that the systems and methods described may be applied in other contexts. In other words, it may be advantageous to stabilize corneal structure with a cross-linking agent as described above as a part of any treatment.

While the invention is susceptible to various modifications and alternative forms, specific embodiments and methods thereof have been shown by way of example in the drawings and are described in detail herein. It should be understood, however, that it is not intended to limit the invention to the particular forms or methods disclosed, but, to the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention. 

1-28. (canceled)
 29. A system for controlling cross-linking in corneal tissue, comprising: a cross-linking system configured to apply a cross-linking agent to a cornea and activate the applied cross-linking agent to cause cross-linking in corneal fibrils of the cornea; a monitoring system configured to determine an amount of cross-linking in the cornea; and a delivery device configured to provide a gas mixture at a surface of the cornea, the gas mixture determining an O₂ content for activation of the cross-linking agent in the cornea, the delivery device being configured to selectively increase or decrease the O₂ content provided at the surface of the cornea to control the rate of cross-linking in response to the amount of cross-linking in the cornea determined by the monitoring system.
 30. The system according to claim 29, wherein the monitoring device includes a controller.
 31. The system according to claim 29, further comprising a cross-linking-agent source storing the cross-linking agent, the cross-linking agent being Riboflavin or Rose Bengal.
 32. The system according to claim 29, wherein the delivery device includes a ring that includes one or more ports that direct the gas mixture at the cornea.
 33. The system according to claim 29, wherein the delivery device includes an enclosure that is pressurized with the gas mixture against the surface of the cornea.
 34. The system according to claim 33, wherein the gas mixture within the enclosure establishes a steady state of O₂ above the surface of the cornea.
 35. The system according to claim 33, wherein the enclosure includes one or more walls, at least a portion of the one or more walls being configured to allow microwave energy or photoactivating light generated by the cross-linking system to pass through the enclosure to activate the cross-linking agent.
 36. The system according to claim 33, wherein the cross-linking system includes an activator that provides microwave energy or photoactivating light to the cornea, the activator being disposed within the enclosure.
 37. The system according to claim 29, wherein the cross-linking system includes an activator configured to provide microwave energy to the cornea that activates the cross-linking agent.
 38. The system according to claim 29, wherein the cross-linking system includes a light source configured to provide photoactivating light to the cornea that activates the cross-linking agent.
 39. The system according to claim 29, wherein the monitoring system employs polarimetry to measure corneal birefringence and to determine the amount of cross-linking in the cornea in real time.
 40. The system according to claim 29, wherein the delivery device initially provides a first gas mixture that determines a first O₂ content in the cornea and, in response to the amount of cross-linking determined by the monitoring system, provides a second gas mixture that determines a second O₂ content in the cornea, the first gas mixture and the second gas mixture providing different amounts of O₂, the delivery device providing dynamic control of cross-linking in the cornea.
 41. The system according to claim 40, wherein the delivery device is configured such that one of the first and second gas mixtures consists of one or more non-O₂ gases.
 42. The system according to claim 29, wherein the monitoring system is configured to determine the stiffness of the corneal tissue.
 43. The system according to claim 42, wherein the monitoring system is configure to determine the stiffness of the corneal tissue by employing polarimetry to measure a corneal birefringence.
 44. A system for controlling cross-linking in corneal tissue, comprising: a vessel that stores a cross-linking agent under O₂ pressure that increases the equilibrium concentration of O₂ for the cross-linking agent causing the cross-linking agent to be saturated or supersaturated with O₂; and a cross-linking system configured to apply the cross-linking agent to corneal fibrils in the corneal tissue, the saturation or supersaturation of the cross-linking agent increasing the O₂ content in the corneal tissue when the cross-linking agent is applied, the cross-linking system being further configured to activate the applied cross-linking agent, the activated cross-linking agent causing cross-linking in the corneal fibrils.
 45. The system according to claim 44, further comprising a cross-linking-agent source storing the cross-linking agent, wherein the cross-linking agent is Riboflavin or Rose Bengal.
 46. The system according to claim 44, wherein the cross-linking system includes an activator configured to activate the cross-linking agent with microwave energy or photactivating light.
 47. A system for controlling cross-linking in corneal tissue, comprising: an applicator configured to apply a cross-linking agent to a cornea; an activator configured to activate the cross-linking agent causing cross-linking in corneal fibrils of the cornea; a delivery device configured to provide a gas mixture at a surface of the cornea, the gas mixture determining an O₂ content for activation of the cross-linking agent in the cornea, the delivery device being configured to selectively increase or decrease the O₂ content provided at the surface of the cornea to control the rate of cross-linking.
 48. The system according to claim 47, wherein the delivery device includes an enclosure that is pressurized with the gas mixture against the surface of the cornea.
 49. The system according to claim 47, wherein the activator is configured to provide microwave energy to the cornea that activates the cross-linking agent.
 50. The system according to claim 47, wherein the activator is configured to provide photoactivating light to the cornea that activates the cross-linking agent.
 51. The system according to claim 47, wherein the applicator includes a syringe. 